A prime suspect in an inquiry into the 2006 death of former Russian security officer Aleksandr Litvinenko has said he had not been given permission by the Russian authorities to give evidence to a London inquiry into the case.

The BBC quoted Dmitry Kovtun on July 27 as saying he needed permission because testifying for London would risk violating an obligation of confidentiality to Russian authorities investigating the case.

The inquiry's head judge Robert Owen said Kovtun's remarks prompt "the gravest suspicion that an attempt is being made to manipulate the situation."

The Russian businessman had been due to appear via video link from Moscow for three days from July 27 but has given a series of reasons for not being able to testify.

Kovtun has been given until July 28 to begin giving testimony.

Kovtun and a fellow Russian, Andrei Lugovoi, are wanted by Britain for allegedly poisoning Litvinenko with a rare radioactive isotope in a London hotel.

While on his deathbed, Litvinenko, a longtime critic of Vladimir Putin, accused the Russian president of ordering his murder.